The circuit converts frequency to voltage by taking the average de value of the pulses from the 7 4121 monostable multivibrator. The one shot is triggered by the positive-going ac signal at the input of the 529 comparator. The amplifier acts as a de filter, and also provides zeroing. This circuit will maintain an accuracy of2% over 5 decades of range.
The input signal to the comparator should be greater than 0.1 V pkpk, and less than 12 V pk-pk for proper operation.